1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus which is the most appropriate for a color printer for forming a color image on an image retainer by electrophotography.
2. Description of the Prior Art
With the wider spread of color image devices such as a color CRT display for a computer and a videotex terminal equipment, demand for color hard copies has increased. This demand for color copies with has also increased due to with the increase in the percentage of the wholly or partially colored copies in various documents in business affairs. Means for making color hard copies is desired to be inexpensive, quick and simple to handle. These conditions are known to be satisfied by a wire dot matrix recorder, an ink jet recorder, a heat transfer recorder, an electrostatic recorder or an electrophotography.
Of these, the electrophotography is advantageous over the other types in that it can easily attain a high resolution of 16 dots/mm, or more, and can record at a high speed,although it is difficult to have a small size because it is equipped with a developing device handling a photosensitive member and powder or liquid.
In one method for that electrophotography, the photosensitive member is exposed to a document through a color filter by a color reader and is developed with a developer of a color complementary to the color of the filter used.
In another method conceivable, a document image is projected through a color filter on photoelectric conversion elements arrayed one-dimensionally like a line sensor, from which an electric output is extracted, and a laser beam is modulated by this signal to make a record in each color by the electrophotography. This is basically identical to the color image treating technique, in a broad sense, which is used in a image signal processing such as a color printing or a color scanner. For forming an image, more specifically, the primaries (B, G and R) are separated to obtain information on three colors, which are arbitrarily combined to perform color reproduction and development.
Not only the method of obtaining the individual color information by separating the color into the primaries but also several methods have been disclosed for obtaining read signals for specified colors by separating a color into two colors and reading them. The latter methods are exemplified by Japanese Patent Laid-Open No. 162755/1981, Japanese Patent Publication No. 44825/1982, as shown in FIG. 20, and the technique reported by the Nippon Telegraph and Telephone Public Corporation at an all-Japan meeting of The Institute of Electronics and Electrical Communication Engineers of Japan in 1982, as shown in FIG. 19.
In another method, a color is separated into two colors through a half mirror and a filter, and color signals are obtained on the basis of those outputs of a one-dimensional photosensor, which in turn are obtained from the two colors, as shown in FIGS. 14 and 15.
According to these methods, a document is separated into two red and blue color components by combining a half mirror and a color filter, and these color components are photoelectrically converted to produce output values by photoelectric conversion means such as CCD. Voltages obtained by normalizing the output values with respect to an output value of white paper are denoted at Vr and Vb, respectively. In the method shown in FIG. 14, the voltages Vr and Vb are plotted on the ordinate and abscissa of the coordinate, respectively. Individual color signals of black, white, blue-green, purple, red-orange and yellow are obtained as the outputs. In the method shown in FIG. 15, individual color signals of black, white, blue, green and red are extracted as hue signals by plotting (Va+Vb) as a luminance signal on the ordinate and (logVa-logVb) as a hue signal on the abscissa.
In the method of obtaining the individual color information by projecting a document image on the photoelectric conversion elements through the filter to separate it into the two or three colors, the photoelectric conversion elements are provided for the color components, respectively. These photoelectric conversion elements of the respective color components will raise a problem if they are relatively deviated. This is because the positionally deviated picture elements will produce erroneous color signals. Even if those deviations occur at a portion or end of the image, the color image forming apparatus is accompanied by a problem that a color having no relation to the document image is reproduced to irritate the sense of vision uncomfortably.